Currently, conventional motion picture theater screens are flat and perforated, and the main speaker units which project the sound tracks on motion picture films are placed behind such perforated screens at about two thirds the height of the screen. The principal advantage of such placement is felt to be that it enhances the illusion that the origin of those frequency components of the sound program which contain directional information, particularly vocal parts, are emanating from the actors visual images on the screen. The perforated screen is acoustically transparent to such lower frequencies, such that there is little loss in sound quality at these frequencies. However, at higher frequencies, the perforated screen becomes increasingly reflective acoustically. The reflected energy can be re-reflected by surfaces behind the screen, in some theaters, thereby altering the high frequency response and sound localization and confusing stereo imaging.
One of the disadvantages of flat screens is that off-axis light rays from the projector tend to be reflected divergingly away from the audience resulting in low perceived brightness in the corners of the screen to viewers sitting off the center line of the theater. This problem is compounded by the fact that off-axis rays must travel somewhat longer distances to the flat screen than axial rays. To overcome this problem, screens cylindrically curved about a vertical axis have been devised which, to an extent, increase the brightness of corner areas of the images on the screen to viewers sitting toward the sides of the theater. An inherent disadvantage of perforated screens, whether flat or curved, is that light rays which enter the perforations of the screen are not available for reflection toward the viewers. Thus, perforated screens are not efficient reflectors of light.
In order to improve the reflection efficiency of movie screens, the Stewart Filmscreen Corporation of Torrance, California has developed what is referred to as a large compound curved screen. The screen itself is an unperforated sheet of a vinyl material which closes a side of an enclosure. A vacuum is pulled on the enclosure which draws the screen material into an externally concave spherical shape. With a film projector at the center of the sphere, there is virtually no variation in the ray distance from the projector to the screen. And while a vertically cylindrical screen improves the lateral brightness consistency, the Stewart screen improves both the lateral and vertical brightness consistency. A significant additional improvement is that since the screen material is unperforated, an increased reflectance surface is available for a given screen area relative to a perforated screen. Thus, a lower projection bulb intensity is required for a given screen brightness compared to perforated screens.
Unfortunately, while the Stewart screen has significantly improved the visual presentation of films, it has created somewhat of a problem for the audio component. The size of the preferred screen enclosure does not provide sufficient room for conventional speaker enclosures behind the screen. Even if such space were available, the unperforated nature of the screen material would severely restrict the transmission of higher frequency portions of the sound tracks.